Transconductor systems

ABSTRACT

One example includes a transconductor system. The system includes a first transconductance amplifier that generates a control current in response to a first input voltage. The system also includes a second transconductance amplifier that generates an output signal in response to a second input voltage. The system further includes an intermediate amplifier that generates a control voltage in response to the control current and a third input voltage. The control voltage can be provided to the first and second transconductance amplifiers to set a transconductance of each of the first and second transconductance amplifiers to be approximately equal.

CROSS REFERENCE TO RELATED APPLICATIONS

Under 35 U.S.C. § 120, this continuation application claims benefits of and priority to U.S. patent application Ser. No. 15/808,002, filed on Nov. 9, 2017, the entirety of which are hereby incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to electronic systems, and more specifically to a transconductor system.

BACKGROUND

A transconductor (e.g., transconductance amplifier) is a circuit device that converts an input signal (e.g., an input voltage) to an output signal (e.g., an output current). A transconductor can have a transconductance that defines a gain of the transconductor, such that the transconductance can define an amplitude of the output signal in response to the input signal. Transconductors can be implemented in a variety of circuit applications, such as in power supply systems. For example, a given power supply system can generate an output voltage based on an input voltage, with both the input and output voltages being potentially highly variable. For a buck converter, as an example, the input voltage can be greater than the output voltage. The power supply system can include an input-current control loop and a differential current balancing loop (e.g., dual-phase) that can exhibit a loop gain that can have a dependence on the input voltage and/or the output voltage. The loop gain can also be affected by a response time, such as in response to transient changes to the input voltage and/or the output voltage, and can also be affected by a bandwidth of the respective amplitudes of the input voltage and/or the output voltage.

SUMMARY

One example includes a transconductor system. The system includes a first transconductance amplifier that generates a control current in response to a first input voltage. The system also includes a second transconductance amplifier that generates an output signal in response to a second input voltage. The system further includes an intermediate amplifier that generates a control voltage in response to the control current and a third input voltage. The control voltage can be provided to the first and second transconductance amplifiers to set a transconductance of each of the first and second transconductance amplifiers to be approximately equal.

Another example includes a transconductor system. The system includes a first transconductance amplifier that generates a control current in response to a first input voltage and a second transconductance amplifier that generates an output signal in response to a second input voltage. The system also includes an intermediate amplifier that generates a control voltage in response to the control current and a third input voltage. The control voltage can be provided to the first and second transconductance amplifiers to set a transconductance of the transconductor system to be proportional to a ratio of the third input voltage and the first input voltage.

Another example includes a power regulator system. The system includes a rectifier that converts an AC input voltage to an input voltage and a buck regulator that generates an output voltage in response to a power regulation signal. The system further includes a transconductor system that generates the regulation signal in response to the input voltage and the output voltage. The transconductor system can have a transconductance that is proportional to a ratio of the input voltage and the output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a transconductor system.

FIG. 2 illustrates an example of a power supply system.

FIG. 3 illustrates an example of a transconductor circuit.

DETAILED DESCRIPTION

This disclosure relates generally to electronic systems, and more specifically to a transconductor system. The transconductor system can be configured to generate an output signal in response to a set of input signals. The transconductor system can include a first transconductance amplifier that is configured to generate a control current in response to a first input voltage. As an example, the first input voltage can correspond to an output voltage that is provided from a buck converter in a power supply system that includes the transconductor. The output voltage can thus be provided to the first transconductance amplifier via a voltage divider to provide a pair of inputs to the first transconductance amplifier. The transconductor system can also include a second transconductance amplifier that is configured to generate an output signal in response to a second input voltage. As an example, the second input voltage can correspond to a current monitoring voltage corresponding to an amplitude of an input current associated with an input voltage that is provided to the power supply system that includes the transconductor, such as from a rectifier. The current monitoring voltage can thus be provided to the second transconductance amplifier along with a predetermined reference voltage corresponding to a current amplitude to which the amplitude of the input current is desired to be regulated.

The transconductor system further includes an intermediate amplifier that can correspond to a voltage amplifier that is configured to generate a control voltage in response to the control current and a third input voltage. As an example, the third input voltage can correspond to the input voltage that is provided to the power supply system. The control voltage can thus be provided to each of the first and second transconductance amplifiers to control a transconductance of each of the first and second transconductance amplifiers. As an example, the first and second transconductance amplifiers can be fabricated from fabrication-matched components (e.g., with respect to associated transistors), such that the control voltage can set the transconductance of the first and second transconductance amplifiers to be approximately equal. The second transconductance amplifier can generate a regulation output signal that can be provided to a power regulator (e.g., a buck regulator), such that the transconductor system can have a transconductance that is proportional to a ratio of the input voltage (e.g., the third input voltage) and the output voltage (e.g., the first input voltage) to regulate the amplitude of the input current. Accordingly, the control loop of the power regulator system, having a loop gain that is proportional to a ratio of the output voltage and the input voltage, can be controlled via the transconductor system, having a transconductance that is proportional to a ratio of the input voltage and the output voltage, to provide power regulation that is absent a dependence on the amplitude of the input voltage or the output voltage.

FIG. 1 illustrates an example of a transconductor system 10. The transconductor system 10 can be implemented in any of a variety of circuit applications that require converting a voltage to an output signal, such as an output current. In the example of FIG. 1, the transconductor system 10 generates a regulation output signal, which is demonstrated as a current I_(REG) which can create a voltage V_(REG). As an example, the transconductor system 10 can be implemented in a power supply system (e.g., a buck power supply system), such as for charging a battery of an electronic device.

The transconductor system 10 includes a first transconductance amplifier 12, a second transconductance amplifier 14, and an intermediate amplifier 16. The first transconductance amplifier 12 is configured to generate a control current I_(CTRL) in response to a first input voltage V₁. As an example, the first input voltage V₁ can correspond to an output voltage that is provided from a buck converter in a power supply system that includes the transconductor system 10. For example, the first input voltage V₁ can be provided to the first transconductance amplifier 12 via a voltage divider (not shown) to provide two proportional voltages that are associated with the first input voltage V₁.

The second transconductance amplifier 14 is configured to generate the current I_(REG) in response to a second input voltage V₂, and in response to the predetermined reference voltage V_(REF). As an example, the second input voltage V₂ can correspond to a current monitoring voltage associated with an amplitude of a current associated with a third input voltage V₃ that can correspond to an input voltage that is provided to the associated power supply system (e.g., from a rectifier). The predetermined reference voltage V_(REF) can correspond to a current amplitude to which the amplitude of the input current (e.g., of the third input voltage V₃) is desired to be regulated. Therefore, the second transconductance amplifier 14 can generate the current I_(REG) which can create the voltage V_(REG) based on an amplitude difference between the second input voltage V₂ and the predetermined reference voltage V_(REF).

The intermediate amplifier 16 can be configured as a voltage amplifier that is configured to generate a control voltage V_(CTRL) in response to the control current I_(CTRL) and in response to the third input voltage V₃ that can correspond to the input voltage of the associated power supply system. In the example of FIG. 1, the control voltage V_(CTRL) is provided to each of the first and second transconductance amplifiers 12 and 14 to control a respective transconductance of each of the first and second transconductance amplifiers 12 and 14. As an example, the first and second transconductance amplifiers 12 and 14 can be fabricated from respective fabrication-matched components (e.g., with respect to associated transistors, such as including differential pairs), such that the control voltage V_(CTRL) can set the transconductance of the first and second transconductance amplifiers 12 and 14 to be approximately equal. For example, the first transconductance amplifier 12 and the intermediate amplifier 16 can be configured in a feedback arrangement, such that the amplitude of the control voltage V_(CTRL) is adjusted to regulate the control current I_(CTRL) relative to the third input voltage V₃. The adjustment of the control voltage V_(CTRL) can thus set the transconductance of the second transconductance amplifier 14 to be approximately equal to the first transconductance amplifier 12, such that the second transconductance amplifier 14 can generate the voltage V_(REG) based on the second input voltage V₂ relative to the predetermined reference voltage V_(REF) and based on the transconductance of the first and second transconductance amplifiers 12 and 14.

Based on the operation of the transconductor system 10, the transconductor system 10 can have a transconductance that is proportional to a ratio of the first input voltage V₁ and the third input voltage V₃. As described previously, the power regulation voltage V_(REG) that can be provided to a power regulator (e.g., a buck regulator) that can have a control loop that is proportional to a ratio of an output voltage and an input voltage. By providing the third input voltage V₃ as the input voltage from the buck power regulator, and by providing the first input voltage V₁ as the output voltage that is provided to the power regulator system (e.g., from a rectifier), the control loop of the power regulator system can provide power regulation that is absent a dependence on the amplitude of the input voltage or the output voltage of the power regulation system, as described herein.

FIG. 2 illustrates an example of a power supply system 50. The power supply system 50 can correspond to an AC-DC power supply system that is configured to convert an AC power voltage, demonstrated in the example of FIG. 2 as a voltage V_(AC), into a DC power voltage, demonstrated in the example of FIG. 2 as an output voltage V_(OUT). As an example, the power supply system 50 can be implemented in a charging circuit for charging a battery of a portable electronic device.

The power supply system 50 includes a rectifier 52 that is configured to rectify the AC input voltage V_(AC) to generate a DC input voltage V_(IN) (hereinafter “input voltage V_(IN)”) having an input current I_(IN). The rectifier 52 can also include a number of other power conditioning functions (e.g., filtering and/or step-down amplification) in addition to rectifying the AC input voltage V_(AC) to generate the input voltage V_(IN). The input voltage V_(IN) is provided to a transconductor system 54 and to a current monitor 56. As an example, the current monitor 56 is configured to monitor an amplitude of the input current I_(IN), and is thus configured to generate a monitoring voltage W_(CM) that is provided to the transconductor system 54. The current monitoring voltage W_(CM) can thus have an amplitude that is proportional to the input current I_(IN).

The power supply system 50 further includes a buck converter 58 that is configured to generate an output voltage V_(OUT) based on a power regulation voltage V_(REG) that is generated by the transconductor system 54. The buck converter 58 can be configured as any of a variety of switching buck converters that is configured to provide the output voltage V_(OUT) as a DC voltage at a lesser amplitude than the power regulation voltage V_(REG). The output voltage V_(OUT) is provided to the transconductor system 54 in a feedback manner, as described in greater detail herein.

The transconductor system 54 can be configured to regulate a current associated with the input voltage V_(IN) based on providing the power regulation voltage V_(REG). As an example, the transconductor system 54 can be configured substantially similar to the transconductor system 10 in the example of FIG. 1. For example, the first input voltage V₁ of the transconductor system 10 can correspond to the output voltage V_(OUT) in the example of FIG. 2, and the third input voltage V₃ of the transconductor system 10 can correspond to the input voltage V_(IN) in the example of FIG. 2. Therefore, the transconductor system 54 can be configured to provide the power regulation voltage V_(REG) at a transconductance that is proportional to a ratio of the input voltage V_(IN) and the output voltage V_(OUT). Additionally, the second input voltage V₂ of the transconductor system 10 can correspond to the monitoring voltage V_(CM), such that the transconductance can be set to provide the power regulation voltage V_(REG) based on the amplitude of the current V_(IN) based on the predetermined reference voltage V_(REF), as described previously.

FIG. 3 illustrates an example of a transconductor circuit 100. The transconductor circuit 100 can be implemented in any of a variety of circuit applications that require converting a voltage to an output signal. In the example of FIG. 3, the transconductor circuit 100 generates a pair of regulation output currents I_(REG1) and I_(REG2) that can be converted to voltages V_(REG1) and V_(REG2) as a differential output signal. The transconductor circuit 100 can correspond to the transconductor system 54 in the example of FIG. 2. Therefore, reference is to be made to the example of FIG. 2 in the following description of the example of FIG. 3.

The transconductor circuit 100 includes a first transconductance amplifier 102, a second transconductance amplifier 104, and an intermediate amplifier 106. The first transconductance amplifier 102 is configured to generate a control current I_(CTRL) in response to the output voltage V_(OUT). In the example of FIG. 3, the output voltage V_(OUT) is provided to a voltage-divider formed by a set of resistors R₁, R₂, and R₃. The voltage-divider provides a first divided voltage V_(OUT1) between the resistors R₁ and R₂ and a second divided voltage V_(OUT2) between the resistors R₂ and R₃, with the resistor R₃ being coupled to a low-voltage rail (e.g., ground). The first divided voltage V_(OUT1) is provided to a gate of a P-channel metal-oxide semiconductor field-effect transistor (MOSFET, hereinafter “P-FET”) P₁, and the second divided voltage V_(OUT2) is provided to a gate of a P-FET P₂. The P-FETs P₁ and P₂ are arranged as a differential pair having a common source connection, with the P-FET P₁ being coupled to a diode-connected N-FET N₁ and the P-FET P₂ being coupled to an N-FET N₂ that has a common gate coupling to the N-FET N₁. The N-FETs N₁ and N₂ are coupled at a source to the low-voltage rail, with the N-FET N₂ having an output node 107 at the drain that provides the control current I_(CTRL). In addition, the first transconductance amplifier 102 includes a control P-FET P₃ that interconnects the common source of the P-FETs P₁ and P₂ and a high rail voltage V_(DD). As described in greater detail herein, the control P-FET P₃ is controlled to conduct a tail current I_(GM1) that sets a transconductance of the first transconductance amplifier 102.

Based on the arrangement of the first transconductance amplifier 102, the first transconductance amplifier 102 is configured to generate the control current I_(CTRL) based on the amplitude of the output voltage V_(OUT) and based on a transconductance set by the tail current I_(GM1). Based on the voltage-divider formed by the resistors R₁, R₂, and R₃, the divided voltages V_(OUT1) and V_(OUT2) each have an amplitude that is proportional to the amplitude of the output voltage V_(OUT). Additionally, the first transconductor 102 has a transconductance (gm) that is variable based on the amplitude of the tail current I_(GM1). Accordingly, the control current I_(CTRL) has an amplitude that is based on the amplitude of the output voltage V_(OUT) and the transconductance set by the tail current I_(GM1).

The second transconductance amplifier 104 is configured to generate the regulation current I_(REG1) and I_(REG2) (which can be used to generate respective first and second regulation voltages V_(REG1) and V_(REG2)) in response to the voltage V_(CM). In the example of FIG. 3, the predetermined reference voltage V_(REF) is provided to a gate of a P-FET P₄, and the voltage V_(CM) is provided to a gate of a P-FET P₅. The P-FETs P₄ and P₅ are arranged as a differential pair having a common source connection, with the P-FET P₄ being coupled to a current source 108 that is interconnected by a first output node 110 on which the first regulation current I_(REG1) (generating the first regulation voltage V_(REG1)) is provided, and the P-FET P₅ being coupled to a current source 112 that is interconnected by a second output node 114 on which the second regulation current I_(REG2) (generating the second regulation voltage V_(REG2)) is provided. The current sources 108 and 112 interconnect the first and second output nodes 110 and 114 to the low-voltage rail. In addition, the second transconductance amplifier 104 includes a control P-FET P₆ that interconnects the common source of the P-FETs P₄ and P₅ and the high rail voltage V_(DD). As described in greater detail herein, the control P-FET P₆ is controlled to conduct a tail current I_(GM2) that sets a transconductance of the second transconductance amplifier 104.

Based on the arrangement of the second transconductance amplifier 104, the second transconductance amplifier 104 is configured to generate the first and second regulation currents I_(REG1) and I_(REG2) (generating the respective regulation voltages V_(REG1) and V_(REG2)) based on the amplitude of the voltage V_(CM) relative to the predetermined reference voltage V_(REF) and based on a transconductance set by the tail current I_(GM2). As described previously, the predetermined reference voltage V_(REF) can correspond to a corresponding current amplitude to which the power supply system regulates the current I_(IN), on which the amplitude of the voltage V_(CM) is based. Additionally, the second transconductor 104 has a transconductance (gm) that is variable based on the amplitude of the tail current I_(GM2). Accordingly, each of the regulation voltages V_(REG1) and V_(REG2) (e.g., the differential voltage V_(REG)) has an amplitude that is based on the amplitude of the voltage V_(CM) relative to the predetermined reference voltage V_(REF) and the transconductance set by the tail current I_(GM2).

As an example, the circuit components of the first and second transconductance amplifiers 102 and 104 can be fabrication matched. As described herein, the term “fabrication matched” with respect to the circuit components can refer to circuit components that are fabricated as approximately identical with respect to size and/or electrical characteristics, and can be fabricated on the same wafer or same part of a wafer to provide for substantially similar fabrication, temperature, and tolerance characteristics and sensitivities. Therefore, the first and second transconductance amplifiers 102 and 104 can exhibit substantially identical performance characteristics. In addition, the P-FETs P₃ and P₆ can likewise be fabrication matched, such that the tail currents I_(GM1) and I_(GM2) can be approximately equal. Accordingly, the first and second transconductance amplifiers 102 and 104 can be controlled by approximately equal tail currents I_(GM1) and I_(GM2), respectively, to provide an approximately equal transconductance.

In the example of FIG. 3, the intermediate amplifier 106 is configured as a voltage amplifier that is configured to generate a control voltage V_(CTRL) on a control node 116 that is provided to the gate of each of the P-FETs P₃ and P₆ to set an amplitude of the respective tail currents I_(GM1) and I_(GM2). The intermediate amplifier 106 includes a first N-FET N₃ and a second N-FET N₄ that are arranged as a differential pair having a common source connection that is coupled to a current source 118 that provides the differential current to the low-voltage rail. The N-FET N₃ is coupled to a diode-connected P-FET P₇ and the N-FET N₄ is coupled to a P-FET P₈ that has a common gate coupling to the P-FET P₇. The P-FETs P₇ and P₈ are coupled at a source to the high rail voltage V_(DD), with the P-FET P₈ having a drain that is coupled to the node 116 on which the control voltage V_(CTRL) is provided.

In the example of FIG. 3, the gate of the N-FET N₃ is coupled to the output node 107 on which the control current I_(CTRL) is provided, and is also coupled to a resistor R_(GM) that interconnects the output node 107 and a node 120. The gate of the N-FET N₄ is coupled to a first resistor R₄ and a second resistor R₅ that are arranged as a voltage-divider with respect to the input voltage V_(IN) to provide a current I₁ through the resistor R₄, such that the gate of the N-FET N₄ is controlled by a voltage that is proportional to the input voltage V_(IN). The resistor R₅ is coupled to the node 120, such that the node 120 interconnects the resistor R₅ and a diode D1 that has a cathode coupled to the low-voltage rail. Therefore, the differential pair of the N-FETs N₃ and N₄ is controlled by the control current I_(CTRL) and the input voltage V_(IN).

Particularly, in the example of FIG. 3, the control current I_(CTRL) sets a voltage amplitude at the gate of the N-FET N₃ via the resistor R_(GM), and the input voltage V_(IN) sets a proportional voltage at the gate of the N-FET N₄ relative to a voltage at the node 120 that is based on the control current I_(CTRL). Thus, the N-FETs N₃ and N₄ are operated as differential pair in the saturation mode of operation based on the input voltage V_(IN) and the control current I_(CTRL). In response, the intermediate amplifier 106 operates to substantially equalize the gate voltages of the N-FETs N₃ and N₄, and thus to equalize the current flow through each of the N-FETs N₃ and N₄. Therefore, the amplitude of the control voltage V_(CTRL) is adjusted based on the gate voltage difference between the N-FET N₃ and N-FET N₄ that results from the amplitude of the control current I_(CTRL), as provided by the voltage at the node 120 across the resistor R_(GM). Accordingly, the control voltage V_(CTRL) adjusts the activation state of the P-FET P₃ to modify the tail current I_(GM1), and thus the transconductance of the first transconductance amplifier 102, to modify the amplitude of the control current I_(CTRL) in a feedback manner. The change in the amplitude of the control current I_(CTRL) thus adjusts the differential control of the differential pair of the N-FETs N₃ and N₄ to provide a steady-state of operation. Accordingly, the intermediate amplifier 106 and the first transconductance amplifier 102 are arranged in a feedback manner to modify the transconductance of the first transconductance amplifier 102.

As described previously, the circuit components of the first and second transconductance amplifiers 102 and 104 can be fabrication matched, and the P-FETs P₃ and P₆ can likewise be fabrication matched. As a result, the changes in amplitude of the control voltage V_(CTRL) to change the amplitude of the tail current I_(GM1) via the P-FET P₃ can likewise change the amplitude of the tail current I_(GM2) via the P-FET P₆. Accordingly, the change to the transconductance of the first transconductance amplifier 102 via the amplitude of the control voltage V_(CTRL) can likewise result in an approximately identical change in the transconductance of the second transconductance amplifier 104 via the amplitude of the control voltage V_(CTRL). Accordingly, the second transconductance amplifier 104 can provide the differential regulation voltage V_(REG1) and V_(REG2) based on the voltage V_(CM) relative to the predetermined reference voltage V_(REF) at approximately the same transconductance as the first transconductance amplifier 102.

The operation of the transconductor circuit 100 can better be explained mathematically. The transconductance GM₁ of the first transconductance amplifier 102 can be expressed as follows: GM ₁ =I _(CTRL) /∝V _(OUT)  Equation 1

-   -   Where: ∝V_(OUT) corresponds to the proportional relationship         between the voltages V_(OUT1) and V_(OUT2).

The control of the first N-FET N₃ of the intermediate amplifier 106 can thus be expressed as follows: I _(CTRL) *R _(GM) =I ₁ *R ₅ =V _(IN)*(R ₅/(R ₄ +R ₅))  Equation 2 I _(CTRL) =V _(IN)*(R ₅/(R ₄ +R ₅))*(1/R _(GM))  Equation 3 Therefore, the transconductance GM₁ can be expressed as: GM ₁=(V _(IN) /V _(OUT))*(R ₅/(R ₄ +R ₅))*(1/(∝R _(GM))  Equation 4 Accordingly, as demonstrated in Equation 4, the transconductance GM₁ is proportional to a ratio of the input voltage V_(IN) and the output voltage V_(OUT). As described previously, the transconductance GM₂ of the second transconductance amplifier 104 is approximately equal to the transconductance GM₁. Accordingly, the transconductance GM₂ of the second transconductance amplifier 104 can be expressed as: GM ₂ =GM ₁=(V _(IN) /V _(OUT))*(R ₅/(R ₄ +R ₅))*(1/(∝<R _(GM)))  Equation 5

As a result, the transconductor circuit 100 can exhibit a transconductance that is proportional to a ratio of the input voltage V_(IN) and the output voltage V_(OUT). As described previously, the power regulation voltage V_(REG) that can be provided to the buck converter 58 that can have a control loop that is proportional to a ratio of the output voltage V_(OUT) and the input voltage V_(IN). For example, by implementing the transconductor circuit in the input current regulation loop, the normal dependence on V_(OUT) and V_(IN) can be eliminated (e.g., V_(OUT)/V_(IN)*V_(IN)/V_(OUT)=1). Accordingly, the control loop of the power regulator system 50 can provide power regulation that is absent a dependence on the amplitude of the input voltage V_(IN) or the output voltage V_(OUT) of the power regulation system 50. This allows loop gain and bandwidth to be maximized, which enhances the transient response time and mitigates potential overload on the input source. The transconductor circuit 100 further helps simplify stabilizing the power regulator system loop 50 in the presence of various critical frequencies.

What have been described above are examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements. As used herein, the term “includes” means includes but not limited to, and the term “including” means including but not limited to. The term “based on” means based at least in part on. 

What is claimed is:
 1. A power supply circuit comprising: a first transconductance amplifier configured to generate a control current amplified from a first voltage and based on a first transconductance; a second transconductance amplifier configured to generate an output signal amplified from a second voltage and based on a second transconductance; and an intermediate amplifier configured to generate a control voltage based on the control current and a third voltage, the control voltage received by the first and second transconductance amplifiers to equalize the first transconductance and the second transconductance.
 2. The power supply circuit of claim 1, wherein: the first voltage is derived from an output voltage; the second voltage corresponds to an amplitude of an input current; and the third voltage is derived from an input voltage.
 3. The power supply circuit of claim 1, wherein the first transconductance amplifier is configured to set the first transconductance, based on the control voltage, to a ratio of the third voltage over the first voltage.
 4. The power supply circuit of claim 1, wherein the second transconductance amplifier is configured to set the second transconductance, based on the control voltage, to a ratio of the third voltage over the first voltage.
 5. The power supply circuit of claim 1, further comprising: an input terminal coupled to the intermediate amplifier to provide the third voltage; a current monitor circuit having an input coupled to the input terminal, and an output coupled to the second transconductance amplifier to provide the second voltage; and a power converter having an input coupled to the second transconductance amplifier to receive the output signal, and an output fed back to the first transconductance amplifier to provide the first voltage.
 6. The power supply circuit of claim 1, wherein the first transconductance amplifier includes a differential amplifier having: a first input coupled to receive a first portion of the first voltage; a second input coupled to receive a second portion of the first voltage; a controlled current source having a control terminal coupled to receive the control voltage; and an output configured to deliver the control current.
 7. The power supply circuit of claim 6, wherein the controlled current source is configured to adjust the first transconductance to approximate the second transconductance based on the control voltage.
 8. The power supply circuit of claim 1, wherein the second transconductance amplifier includes a differential amplifier having: a first input coupled to receive a reference voltage; a second input coupled to receive the second voltage; a controlled current source having a control terminal coupled to receive the control voltage; and an output configured to deliver the output signal.
 9. The power supply circuit of claim 8, wherein the controlled current source is configured to adjust the second transconductance to approximate the first transconductance based on the control voltage.
 10. The power supply circuit of claim 1, wherein the intermediate amplifier includes: a conductive path to receive the control current; and a differential amplifier having: a first input coupled to the conductive path; a second input coupled to receive a portion of the third voltage; and an output configured to deliver the control voltage.
 11. A power supply circuit comprising: a first transconductance amplifier configured to generate a control current amplified from an output voltage and based on a first transconductance; a second transconductance amplifier configured to generate a regulation signal amplified from a sensed voltage proportional to an input current and based on a second transconductance; and an intermediate amplifier configured to generate a control voltage based on the control current and an input voltage, wherein the first and second transconductance amplifiers are configured to adjust the first and second transconductance based on the control voltage, wherein the first transconductance amplifier is configured to set the first transconductance to a ratio of the input voltage over the output voltage.
 12. The power supply circuit of claim 11, wherein: the first transconductance amplifier includes a first controlled current source configured to adjust, based on the control voltage, the first transconductance to approximate the second transconductance; and the second transconductance amplifier includes a second controlled current source configured to adjust, based on the control voltage, the second transconductance to approximate the first transconductance.
 13. The power supply circuit of claim 11, wherein the intermediate amplifier includes: a conductive path to receive the control current; and a differential amplifier having: a first input coupled to the conductive path; a second input coupled to receive a portion of the input voltage; and an output configured to deliver the control voltage.
 14. The power supply circuit of claim 11, wherein the second transconductance amplifier is configured to set the second transconductance to a ratio of the input voltage over the output voltage.
 15. A power supply circuit comprising: a first transconductance amplifier having a first controlled current source configured to adjust a first transconductance based on a control voltage, the first transconductance amplifier configured to generate a control current by amplifying an output voltage with the first transconductance; a second transconductance amplifier having a second controlled current source configured to adjust a second transconductance based on the control voltage, the second transconductance amplifier configured to generate a regulation signal by amplifying a sensed voltage proportional to an input current with the second transconductance; and an intermediate amplifier configured to generate the control voltage based on the control current and an input voltage, wherein the first controlled current source is configured to adjust the first transconductance to a ratio of the input voltage over the output voltage.
 16. The power supply circuit of claim 15, further comprising: an input terminal coupled to the intermediate amplifier to provide the input voltage; a current monitor circuit having an input coupled to the input terminal, and an output coupled to the second transconductance amplifier to provide the sensed voltage; and a power converter having an input coupled to the second transconductance amplifier to receive the regulation signal, and an output fed back to the first transconductance amplifier to provide the output voltage.
 17. The power supply circuit of claim 15, wherein the second controlled current source is configured to adjust the second transconductance to a ratio of the input voltage over the output voltage.
 18. The power supply circuit of claim 15, wherein the intermediate amplifier includes: a conductive path to receive the control current; and a differential amplifier having: a first input coupled to the conductive path; a second input coupled to receive a portion of the input voltage; and an output configured to deliver the control voltage. 